1. Field of the Invention
The present invention is directed to stable aqueous solutions of high efficiency phenol-formaldehyde resins and to a process for preparing the solutions.
2. Description of the Prior Art
A high efficiency single phase aqueous phenol-formaldehyde resin is one which has low free phenol and low saligenin (ortho-hydroxy benzyl alcohol) and can be aptly described as one in which a high percentage of the organic moiety of the aqueous resin is retained when the resin is cured. Free phenol and saligenin volatilize in the curing of the resin, reducing the efficiency and the performance of the resin in various bonding applications. A high efficiency resin is extremely valuable to the industry since it results in a greater economic advantage over conventional resins. Even more important is the increase in performance of the resin in bonding applications. However, one of the unfortunate drawbacks with a high efficiency single phase aqueous phenol-formaldehyde resin is the instability of the resin which results in the formation of a solid or crystal phase. This crystal phase is extremely difficult to dissolve and prevents uniform application of the resin to substrates. Heating the resin to elevated temperatures will help to re-dissolve the crystal phase, but, unfortunately, such exposure to high temperatures will also advance the resin and affect its dilutability characteristics thereby affecting its application performance. Filtering out the crystals is uneconomic, reduces the efficiency of the resin and impairs the bonding characteristics.
The prior art (U.S. Pat. No. 3,428,593) teaches stable, high efficiency, single phase, aqueous phenol formaldehyde resins and a process for making them by reacting phenol and formaldehyde under controlled reaction conditions and a controlled excess of free formaldehyde. The resin is first prepared by reacting phenol and formaldehyde in certain critical proportions in the presence of a critical proportion of a basic catalyst. The reaction is continued until an end point of less than 5 weight percent of free phenol is obtained based on the weight of the phenol-formaldehyde resin solids. The free formaldehyde content is then adjusted to at least 3.0 weight percent by the post addition of formaldehyde thereto. The addition of this formaldehyde keeps the resin stable by driving the equilibrium reaction of formaldehyde and bis (4-hydroxy-3, 5-dihydroxymethylphenyl) methane in the direction of forming at least the mono-hemiformal of bis (4-hydroxy-3, 5-dihydroxymethylphenyl) methane, to prevent the formation of crystals of bis (4-hydroxy-3, 5-dihydroxymethylphenyl) methane. This method of preparing stable high efficiency aqueous phenol formaldehyde resins was a valuable advance in the art. However, the free phenol and free formaldehyde content of such resins are relatively high and their presence in the effluents from the resin applicator's plant can contribute to atmospheric pollution.
A need therefore exists for high efficiency stable, single phase, aqueous phenol-formaldehyde resins which contain low concentrations of phenol and formaldehyde and thus cause substantially less pollution of the atmosphere than prior art resins.